Edge bevel removal of copper from silicon wafers

ABSTRACT

Chemical etching methods and associated modules for performing the removal of metal from the edge bevel region of a semiconductor wafer are described. The methods and systems apply liquid etchant in a precise manner at the edge bevel region of the wafer under viscous flow conditions, so that the etchant is applied on to the front edge area and flows over the side edge and onto the back edge in a viscous manner. The etchant thus does not flow or splatter onto the active circuit region of the wafer. An edge bevel removal embodiment involving that is particularly effective at obviating streaking, narrowing the metal taper and allowing for subsequent chemical mechanical polishing, is disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority as a continuation-in-part applicationunder 35 U.S.C. §120 from co-pending U.S. patent application Ser. No.09/557,668, filed Apr. 25, 2000, now U.S. Pat. No. 6,309,981, naming S.Mayer et al. as inventors, and titled “EDGE BEVEL REMOVAL OF COPPER FROMSILICON WAFERS.” This application is also related to U.S. patentapplication Ser. No. 09/557,695 naming Mayer et al. as inventors, andtitled “ETCHANT MIXING SYSTEM FOR EDGE BEVEL REMOVAL OF COPPER FROMSILICON WAFERS” and U.S. patent application Ser. No. 09/558,249 namingMayer et al. as inventors, and titled “WAFER CHUCK FOR USE IN EDGE BEVELREMOVAL OF COPPER FROM SILICON WAFERS,” both filed on Apr. 25, 2000.These patent applications, as well as any patents, patent applicationsand publication referenced are hereby incorporated by reference in theirentireties and for all purposes.

BACKGROUND OF THE INVENTION

This invention relates to technology for removing unwanted metal fromsemiconductor wafers. More particularly, it pertains to methods forremoving unwanted metal, particularly metal in the edge bevel region,using liquid etchants, as well as apparatus modules for performing suchremoval.

Damascene processing is a method for forming metal lines on integratedcircuits. It is often a preferred method because it requires fewerprocessing steps than other methods and offers a higher yield. InDamascene processing, as well as other integrated circuit manufacturingprocesses, the conductive routes on the surface of the circuit aregenerally formed out of a common metal, traditionally aluminum. Copperis a favored metal because of its higher conductivity andelectromigration resistance when compared to aluminum, but copperpresents special challenges because it readily diffuses into siliconoxide and reduces its electrical resistance at very low doping levels.During integrated circuit fabrication, the conductive metal is needed onthe active circuit region of the wafer, i.e., the main interior regionon the front side, but is undesirable elsewhere.

In a typical copper Damascene process, the formation of the desiredconductive routes generally begins with a thin physical vapor deposition(PVD) of the metal, followed by a thicker electrofill layer (which isformed by electroplating). The PVD process is typically sputtering. Inorder to maximize the size of the wafer's useable area (sometimesreferred to herein as the “active surface region”) and thereby maximizethe number of integrated circuits produced per wafer), the electrofilledmetal must be deposited to very near the edge of the semiconductorwafer. Thus, it is necessary to allow physical vapor deposition of themetal over the entire front side of the wafer. As a byproduct of thisprocess step, PVD metal typically coats the front edge area outside theactive circuit region, as well as the side edge, and to some degree, thebackside.

Electrofill of the metal is much easier to control, since theelectroplating apparatus can be designed to exclude the electroplatingsolution from undesired areas such as the edge and backside of thewafer. One example of plating apparatus that constrains electroplatingsolution to the wafer active surface is the SABRE™ clamshellelectroplating apparatus available from Novellus Systems, Inc. of SanJose, Calif. and described in U.S. Pat. No. 6,156,167 “CLAMSHELLAPPARATUS FOR ELECTROCHEMICALLY TREATING SEMICONDUCTOR WAFERS,” by E.Patton et al., and filed Nov. 13, 1997, which is herein incorporated byreference in its entirety.

The PVD metal remaining on the wafer edge after electrofill isundesirable for various reasons. One reason is that PVD metal layersleft at the edge after CMP are not suitable for subsequent layer metaldeposition on top of them (e.g. subseqent dielectric layer will notadhere well to the PVD copper base layer if it is not removed). Also,the PVD layers are thin and tend to flake off during subsequenthandling, thus generating undesirable particles. This can be understoodas follows. At the front side edge of the wafer, the wafer surface isbeveled. Here the PVD layers are not only thin, but also unevenlydeposited. Thus, they do not adhere well. Adhesion of subsequentdielectric layers onto such thin metal is also poor, thus introducingthe possibility of even more particle generation. By contrast the PVDmetal on the active interior region of the wafer is simply covered withthick, even electrofill metal and planarized by CMP down to thedielectric. This flat surface, which is mostly dielectric, is thencovered with a barrier layer substance such as SiN, that both adhereswell to the dielectric and aids in the adhesion of subsequent layers.Another reason to remove the residual PVD metal layers in the wafer edgearea is that the barrier layers underneath them are also thin anduneven, which may allow migration of the metal into the dielectric. Thisproblem is especially important when the metal is copper.

To address these problems, semiconductor equipment may have to allowetching of the unwanted residual metal layers. Various difficulties willbe encountered in designing a suitable etching system.

One of the main difficulties involves the precise application of theetchant to the edge bevel region without allowing it to contact theactive circuit region of the wafer. Physical shielding of the activecircuit region is an option, but it is undesirable because contactingthe wafer in this manner causes particle generation from the surface ofthe wafer. In addition, it is highly desirable to apply the etchant in avery narrow, confined region at the outer boundary of the wafer, so thatthe interior active circuit region is defined as expansively aspossible. Other difficulties in designing an etching method and systeminclude precise alignment of the wafer on the wafer chuck for rotation,proper pre-wetting, rinsing and drying procedures, and adequate clampingof the wafer in situations where undesired movement is possible. Sincebackside etching of the wafer is often necessary and desirable at thetime of edge bevel removal (EBR), an invention addressing these needsshould also be able to perform the back side etch.

Additional problems include the fact that etchant may splash back fromthe walls of the EBR module, thus causing unwanted oxidation(“streaking”) on the wafer surface. Nozzle orifices for dispensing theetchant are difficult to manufacture to precise desired diameters, andany variance in this diameter can result in significantly varying exitvelocities from the nozzle. The taper from the region of metalization tothe region without metal may also be quite wide, which is undesirablefor purposes with respect to subsequent copper removal/planarizationsteps (e.g. CMP, electropolishing, electroetching).

SUMMARY OF THE INVENTION

The present invention provides chemical etching methods and associatedmodules for performing the removal of metal from the edge bevel regionof a semiconductor wafer, which includes the front side edge, the sideedge and the back side. The invention provides methods and systems forapplying the etchant in a precise manner at the edge bevel region of thewafer under viscous flow conditions, so that the etchant is applied onto the front edge area and flows over the side edge and onto the backedge in a viscous manner. The etchant thus does not flow or splatteronto the active circuit region of the wafer. An edge bevel removalembodiment that is particularly effective at obviating streaking,narrowing the metal taper and allowing for subsequent chemicalmechanical polishing, is disclosed. One aspect of the invention providesa method for removing metal from the edge bevel area of a semiconductorwafer using a prerinse with deionized water and acid, typically inseparate operations. Subsequently, an etchant is typically deliveredunder viscous flow conditions. The metal to be etched may be depositedcopper. The invention may allow selective removal of metal at a rate ofat least 400 Å per second. Further, the method provides for a liquidetchant delivered onto the edge of the rotating wafer withoutsubstantially contacting any region of the wafer inside of the edgebevel area. In a specific embodiment, the nozzle may be positioned sothat delivery of the liquid etchant has an angular component in thedirection of rotation of the wafer edge (in the direction of the wafertangent) of about 45 degrees. The nozzle may also be positioned so thatdelivery of the liquid etchant has a radial component away from thecenter of the wafer and toward the edge of the wafer of about 25-35degrees. The wafer may be rotated at about 150-400 rpm, for exampleabout 225 or 350 rpm. Preferably, the method reduces the taper width ofthe metal at the edge of the wafer to less than about 300 μm.

These and other features and advantages of the present invention will bedescribed in more detail below with reference to the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an illustration of a semiconductor wafer showing the locationof the edge bevel region that is etched in accordance with thisinvention.

FIG. 1B is a process flow diagram illustrating relevant operationsemployed to form conductive copper lines a Damascene process in thecontext of this invention.

FIG. 1C is a block diagram illustrating a group of modules used to formcopper conductive lines on an integrated circuit.

FIG. 2A is a block diagram illustrating various elements of apost-electrofill module in accordance with one embodiment of thisinvention.

FIG. 2B is a perspective view of a post-electrofill module.

FIG. 2C is a side-view showing a nozzle and nozzle tip sitting on top ofthe wafer plane.

FIG. 2D is a top-view showing a nozzle and nozzle tip sitting on top ofthe wafer plane.

FIG. 2E is a close-up of a nozzle and nozzle tip, the nozzle tip beingreplaceable and precision drilled for the desired orifice size.

FIG. 3A is a process flow diagram illustrating a typical sequence ofoperations employed with a post-electrofill module in accordance with anembodiment of this invention.

FIG. 3B is a process flow diagram depicting a pre-rinse operation inaccordance with an especially preferred embodiment of this invention.

FIG. 4A is a schematic illustration of etchant being delivered to awafer edge bevel via an etchant dispensing nozzle in a manner thatconstrains the etchant to the edge bevel region of the wafer.

FIG. 4B is a top view of a wafer on which etchant is delivered at acontrolled orientation via an etchant delivery nozzle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the present invention, numerousspecific embodiments are set forth in order to provide a thoroughunderstanding of the invention. However, as will be apparent to thoseskilled in the art, the present invention may be practiced without thesespecific details or by using alternate elements or processes. In otherinstances well-known processes, procedures and components have not beendescribed in detail so as not to unnecessarily obscure aspects of thepresent invention.

As indicated, this invention pertains to removal of unwanted coppermetal from an edge bevel region of a semiconductor Wafer. A“semiconductor wafer” as referred to in this invention is asemiconductor substrate at any of the various states of manufacture inthe production of integrated circuits. One standard semiconductor waferdescribed in this invention is 200 mm in diameter, 0.75 mm thick, withan approximate radius of curvature of about 0.15 millimeters (see SEMISpecification M1-0298). Of course, semiconductor wafers of otherdimensions, such as a standard 300 mm diameter silicon wafers, can alsobe processed in accordance with this invention. Note that standardspecifications for a 300 mm diameter wafer may be found in SEMISpecification M1.15-0997.

To facilitate understanding the concepts of this invention, a schematicillustration of a semiconductor wafer is shown in FIG. 1A. As shown,such semiconductor wafer has a top or “front” side 100 and a “backside”101. The wafer also has an interior “active circuit region” 102 whereintegrated circuit devices with associated conductive metal routes areformed. To make maximum use of expensive semiconductor material, thisactive circuit region should constitute a high fraction of the area onthe front side 100 of the wafer. With a 200 mm wafer, the presentinvention allows the interior active surface region to extend theuseable active region to within at least 1.5 and 4 mm of the outerboundary of the wafer. As shown, integrated circuit wafers also includea “front edge” area 103, which is the region on the front of the waferthat lies outside the active circuit region, a “side edge” area 104(sometimes referred to herein as an “edge bevel region”) and a “backedge” area 105. The side edge lies in the area between the front sideand the backside, and the back edge is roughly the area near the outerboundary of the wafer on its backside, approximately analogous to thefront edge area. Unwanted metal such as copper may deposit on regions103, 104, and 105. Some metal may also deposit over the entire backside101. One use of the present invention is to remove unwanted metal fromthese regions without substantially affecting metal deposited on activeregion 102.

A “post-electrofill module” (PEM) or “EBR module” as referred to in thisinvention is a module that is specifically designed to carry out theedge bevel removal (EBR) process, as well as a backside etch (BSE)process, in most cases. It may also perform processes ancillary to theEBR, including pre-rinsing, rinsing, acid washing and drying. Anintegrated-electrofill module as referred to in this invention is amodule that carries out electrofill.

While details of the preferred embodiment may be found below in thisapplication, a short description of a typical Damascene process will nowbe provided to facilitate understanding the context of the presentinvention. A typical Damascene process flow 150 is illustrated in theflowchart of FIG. 1B. Process 150 begins with formation of line paths151 in a previously formed dielectric layer. These line paths may beetched as trenches and vias in a blanket layer of dielectric such assilicon dioxide. They define conductive routes between various deviceson a semiconductor wafer. Because copper or other mobile conductivematerial provides the conductive paths of the semiconductor wafer, theunderlying silicon devices must be protected from metal ions (e.g.,copper) that might otherwise diffuse into the silicon. To accomplishthis, the process includes depositing a thin diffusion barrier layer 152before depositing the metal. Suitable materials for the diffusionbarrier layer include tantalum, tantalum nitride, tungsten, titanium,and titanium tungsten. In a typical embodiment, the barrier layer isformed by a PVD process such as sputtering.

The wafer is now nearly ready to have its line paths inlayed with theelectrofill copper. However, before electrofilling, a conductive surfacecoating must be applied. In the depicted process, this is accomplishedby depositing a copper seed layer on the barrier layer at 153. A. PVDprocess such as sputtering may be employed to this end. The wafer isthen electrofilled at 154 with a thicker layer of copper over the seedlayer, by electroplating using an electroplating solution. The copper isdeposited to a thickness that completely fills the various line paths inthe dielectric layer.

As mentioned, it is desirable to use as much of the wafer surface foractive circuitry as possible. While it is generally a straightforwardmatter to shield unwanted areas from an electroplating solution, thesame kind of shielding cannot be so easily and precisely done with PVD.Thus deposition of PVD copper in some unwanted areas cannot be avoided.This copper must be removed, and this is accomplished by the edge bevelremoval (EBR) and/or backside etch BSE processes of the presentinvention.

With EBR at 155, a copper etchant is applied to the front edge of thewafer in a thin stream. The etchant is preferably applied under viscousflow conditions so that it remains in a thin, viscous layer near thepoint on the wafer where it is applied, and thus avoids splashing theinterior of the wafer. Because the etchant is also generally appliedwith a radial velocity component, and because of the centripetalacceleration effects of the rotating wafer, the thin viscous layer flowsoutward, down over the side edge and a few millimeters onto thebackside, thus accomplishing removal of the PVD copper from all three ofthese areas. More specifics of the EBR process are described below.After EBR, the electroplated copper is planarized, generally bychemical-mechanical polishing (CMP) down to the dielectric at 156 inpreparation for further processing (illustrated at 157), generally theaddition of subsequent dielectric and metalization layers.

FIG. 1C depicts an electrofill system 107 in which the invention mayreside. The specific system includes three separate electrofill modules109, 111 and 113. System 107 also includes three separate postelectrofill modules 115, 117 and 119. Each of these may be employed toperform each of the following functions: edge bevel removal, backsideetching, and acid cleaning of wafers after they have been electrofilledby one of modules 109, 111, and 113. System 107 also includes a chemicaldilution module 121 and a central electrofill bath 123. This is a tankthat holds the chemical solution used as the electroplating bath in theelectrofill modules. System 107 also includes a dosing system 133 thatstores and delivers chemical additives for the plating bath. A chemicaldilution module 135 stores and mixes chemicals to be used as the etchantin the post electrofill modules. A filtration and pumping unit 137filters the plating solution for central bath 123 and pumps it to theelectrofill modules. Finally, an electronics unit 139 provides theelectronic and interface controls required to operate system 107. Unit139 may also provide a power supply for the system.

In operation, a robot including a robot arm 125 selects wafers such as awafer 127 from a wafer cassette such as a cassette 129A or a cassette129B. Robot arm 125 may attach to wafer 127 using a vacuum attachment.

To ensure that wafer 127 is properly aligned on robot arm 125 forprecision delivery to an electrofill module, robot arm 125 transportswafer 127 to an aligner 131. In a preferred embodiment, aligner 131includes alignment arms against which robot arm 125 pushes wafer 127.When wafer 127 is properly aligned against the alignment arms, the robotarm 125 moves to a preset position with respect to the alignment arms.It then reattaches to wafer 127 and delivers it to one of theelectrofill modules such as electrofill module 109. There, wafer 127 iselectrofilled with copper metal. Electrofill module 109 employselectrolyte from a central bath 123.

After the electrofill operation completes, robot arm 125 removes wafer127 from electrofill module 109 and transports it to one of thepost-electrofill modules such as module 117. There unwanted copper fromcertain locations on the wafer (namely the edge bevel region and thebackside) is etched away by an etchant solution provided by chemicaldilution module 121.

Preferably wafer 127 is precisely aligned within post electrofill module117 without making use of aligner 131. To this end, the post electrofillmodules may be provided with an alignment chuck as referenced elsewhereherein. In alternative embodiment, wafer 127 is separately alignedwithin aligner 131 after electrofill and prior to edge bevel removal inmodule 117.

After processing in post electrofill module 117 is complete, robot arm125 retrieves wafer 127 from the module and returns it to cassette 129A.From there the cassettes can be provided to other systems such as achemical mechanical polishing system for further processing.

FIG. 2A schematically illustrates one preferred post-electrofill module220 suitable for use with this invention. FIG. 2B presents such modulein a perspective view. As shown, module 220 includes a chamber 222 inwhich a semiconductor wafer 224 rotates. Wafer 224 resides on a waferchuck 226 which imparts rotational motion to wafer 224. Chamber 222 isoutfitted with a drain and associated drain line 264. The drain allowsthe various liquid streams provided to chamber 222 to be removed forwaste treatment.

A motor 228 controls the rotation of chuck 226. Motor 228 should be easyto control and should smoothly transition between various rotationalspeeds. It may reside within or without chamber 222. In someembodiments, to protect against damage from liquid etchant, motor 228resides outside of chamber 222 and is separated therefrom by a sealthrough which a rotating shaft 227 passes. Any wobble in the shaft onrotation should be small (˜<0.05 millimeters for example) so that thelocation of fluid nozzles with respect to the wafer does not varysubstantially, nor shake the wafer from its center while it is notconfined by alignment or clamping members. Preferably, motor 228 canrapidly accelerate and decelerate (in a controlled fashion) chuck 226and wafer 225 at rotation rates between 0 and about 2000 rpm. The motorspeed and other operations should be controllable by a computer.

Chuck 226 may be of any suitable design that holds wafer 224 in positionduring various rotational speeds. It may also facilitate alignment ofwafer 224 for the etching process. A few particularly preferred examplesof wafer chucks are described in U.S. patent application Ser. No.09/558,249 previously incorporated by reference.

Chamber 222 may be of any suitable design that confines the liquidetchant within its interior and allows delivery of the various fluids towafer 224. It should be constructed of an etchant resistant material andinclude ports and nozzles for the various liquid and gaseous streamsused during etching and cleaning.

Gaseous nitrogen is provided to post electrofill module 220 from asource of nitrogen 230. Preferably, this is a central source of nitrogenavailable to various processes throughout an integrated circuitmanufacturing facility. Nitrogen from source 230 is delivered to chamber222 under the control of a valve 232. The gaseous nitrogen is deliveredinto chamber 222 via a line and nozzle 234 positioned to deliver thenitrogen directly onto wafer 224, preferably at the center of the wafer.This enables blowing dry, particle-free nitrogen at the center, upperface of the wafer. This orientation of the nozzle increases the dryingrate at the wafer center, where the centrifugal forces are small. Othersuitable gas drying sub-systems may be employed as will be appreciatedby those of skill in the art. For example, drying gases other thannitrogen may be employed in some embodiments. Also, the orientation andblowing direction of the nitrogen nozzle may be widely varied.

The next input of interest to module 220 is a source of deionized water236. As with the source of nitrogen 230, the source of deionized water236 preferably originates with a central source within an integratedcircuit fabrication facility. The deionized water is delivered tochamber 222 under the control of a valve 238 and through a delivery lineand nozzle 238. Note that line 238 directs deionized water onto the topof wafer 224. This enables rinsing of the wafer's top side. A preferrednozzle with an internal diameter of between ⅛″ and ¼″ diameter deliversa stream of fluid directed at the center of a wafer. The nozzle heightabove the wafer surface being between 1 to 2 inches. Alternatively anozzle sprays fluid as a thin “fan” that spreads out over the innerthree-quarters of the wafer diameter. Preferably, the thickness of thefan is no more than about one-fifth of the wafer diameter. The spray canimpact the wafer with a velocity in the same direction as the wafer isrotating, or opposite the direction of rotation, or even in bothdirections if the spray fan crosses the wafer center. Preferably, thespray is directed opposite to the direction of rotation to increaseconvective mixing.

A similar deionized water system provides a stream or fan of deionizedwater to the backside of wafer 224. This deionized water is providedfrom a source of deionized water 240, which may be the same as source236. A valve 242 controls the flow of deionized water onto the backsideof wafer 224 via a line and nozzle 244. The nozzle associated with 244may have the same design criteria as just mentioned for nozzle 238. Thegoal is to rinse etchant from the backside of wafer 224.

In a preferred embodiment, an acid rinse is conducted on the front sideof wafer 224. To this end, a source of sulfuric acid 246 providessulfuric acid to a delivery line and nozzle 250. Other acids may be usedas appropriate. Preferably, the source 246 of sulfuric acid is achemical dilution module described in U.S. patent application Ser. No.09/557,695. Preferably, this module includes a valve that controls thedelivery of sulfuric acid to module 220. The flow of sulfuric acid intochamber 222 may be monitored by a mass flow meter 248. Note that in thedepicted embodiment nozzle 250 is oriented to direct sulfuric acid ontothe center of the front side of wafer 224. After the acid is deliveredto the center of the wafer it then spins out into the edge of the waferduring rotation. This solution is applied to remove residual copperoxide which remains after oxidizing (etching) the wafer and aids in theoverall cleaning of the wafer. Only a relatively small amount of acid istypically required (e.g., 0.5 to 2 milliliters/200 mm wafer). After itsapplication, the wafer's front side is rinsed with deionized waterthrough nozzle 238.

Liquid etchant used to remove copper or other unwanted metal fromportions of wafer 224 is provided from a source of liquid etchant 252 asshown. Preferably, this source is provided by the above-mentionedchemical dilution module. The etchant passes through a mass flow meter254 and is delivered to wafer 224 via a line and nozzle 256. Preferably,the etchant is delivered to the edge bevel region of wafer 224 to removePVD copper in that region.

A second liquid etchant stream may be delivered to the backside of wafer224 in order to etch off any copper or other unwanted metal that mayhave been deposited on the backside of wafer 224. As shown, such etchantis delivered from an etchant source 258. Preferably, this is the samesource as 252. In other words, the chemical dilution module providesetchant for both edge bevel removal and backside etch. As shown, etchantfrom source 258 passes through a mass flow meter 260 and through anozzle 262, which directs it onto the backside of wafer 224.

FIG. 2C is a side-view diagram showing some components of the apparatusused to deliver etchant during EBR, in accordance with one embodiment ofthis invention. A nozzle 251 delivers the etchant. It includes a nozzletip 253 that can.be manufactured as part of the same unitary part as thenozzle 251, but it is preferred that the nozzle tip 253 be a separate,replaceable piece, as will be discussed below. Also shown in thisdiagram are one of the chuck alignment pins 255 and the teach waferplane257 where the nozzle tip contacts during set-up and calibration. FIG. 2Dis a top-view showing the same apparatus, including the nozzle 251,nozzle tip 253, wafer support clamp 255 and the wafer 269.

FIG. 2E is a detailed view of a nozzle 271, including a separate nozzletip 273. It has been found that a nozzle orifice size of 0.016 to 0.017inches works well with the preferred embodiments of this invention.However, it can be difficult to precisely and consistently manufacturethe orifice to this size, and orifices of between 0.020 and 0.014 inchesare typically produced in practice. Such variations can change the exitvelocity by a factor of up to 2. Because the quality of the wafersurface (the presence or absence of streaking, spotting, etc.) is strongfunction of exit velocity, the nozzle orifice should be consistentlysized to a precise diameter in each and every apparatus produced. Thishas not been possible with the existing design which relied on a taperedTeflon tube inserted into a soft PVDF nozzle tip to control the dispensestream size. Control of the tubing size and wear associated with thesofter PVDF tip material limits the life and degrades the performance ofthe nozzle. It is therefore desirable to make a separate nozzle tip 273that can be precision drilled and threaded onto the external surface ofthe nozzle piece 271. Preferably, a relatively hard, etchant-resistant,material such as PPS (polyphyenylene sulfide) or various chemicallycompatible ceramic materials (e.g. alumina or zeolite) is employed. FIG.2E also shows the orifice 275 in the nozzle tip, as well as the innerflanged tube end 277 of the nozzle 271, that allow a tight seal betweenthe nozzle 271 and nozzle tip 273.

A specific embodiment of the EBR process is illustrated in FIG. 3A. Asecond, preferred, embodiment will be described below. The depicted EBRprocess 300 can be carried out by a post-electrofill module, such asmodule 220 of FIG. 2A, that is specifically designed to carry out theEBR process. The process begins at 301, with a robot arm placing thewafer on the module chuck for EBR processing. The wafer is placed intothe chuck onto a set of support pins. The wafer is aligned inside thechuck by the vertical alignment pins. The support pins hold the wafer inplace by static friction when the wafer is rotated. A vacuum chuck canalso be used. After the robot arm retracts, deionized water is appliedto the front of the wafer and the wafer is spun at about 150-400 rpm inorder to pre-rinse the wafer of any particles and contaminants left overfrom previous processing steps. See 302. In the simplest embodiment, thepre-rinse operation employs only deionized water—no acid. The pre-rinseoperation takes place 10 to 30 seconds with a flow rate of 200-800ml/minute depending on rinse water temperature, plating chemistry,deionized water flow rate and the rotational speed of the wafer. It issometime desireable to use hot rinse water to accelerate the pre-rinseefficency. Therefore, DI water at from 20 to 50 C can be employeeddepending on the economics of the operations.

The deionized water is then turned off and the wafer continues to spinat a speed of of between about 150-350 rpm , which creates a uniformlythin layer of deionized water (wet-film stabilization). See 303. Thiswet-film stabilization facilitates an even distribution of the etchantover the front side of the wafer. At this time the wafer is in contactwith the support pins only, the alignment pins have rotated away fromthe edge of the wafer and the clamps are at mid-position.

After wet-film stabilization 303, a core feature of the EBR, actualremoval of the edge bevel metal 304 is performed Typically, the wafercontinues to be rotated but 0.5 to 3 seconds is allowed to elapse beforeEBR, in order to allow the deionized water to thin out. Operation 304 istypically carried out at about 150-400 rpm, more preferably—200 to 250rpm for 200 mm wafer and 175 to 225 rpm for 300 mm wafers . This rate ofrotation helps ensure that the entire edge exclusion area is covered bythe EBR etchant. It is preferred to maintain the same pre-rinse and EBRspeed to prevent the wafer from slipping off the support pins andde-centering during any acceleration/deceleration. During theseoperations the wafer is not held at a centering position. The rate ofchange in rotational velocity prior to the EBR step results in movementof the wafer on the support pins. Pins with greater friction coefficentare preferred as long as they do not flake or generate particles.Therefore, this parameter should also be controlled. It is preferredthat the rate exceed about 150 rpm/sec when using typical plasticsupport pins (e.g. PPS or PVDF).

The EBR etchant is typically applied to the surface of the wafer using athin nozzle tube, which has a nozzle opening at or near its end. Whendispensing a small amount of fluid onto a surface as such, three flowregimes can generally result. The first regime is edge beading, wheresurface tension forces dominate the behavior of the fluid, the second isviscous flow, where viscous forces predominate, and the third isinertial, where inertial forces predominate and the fluid tends tospray. As explained below, the preferred flow regime is the viscousflow. In a specific example, an EBR dispense arm is positioned over thewafer edge as described below with reference to FIG. 4B.

In one embodiment, EBR is performed under the following conditions: atotal of about 2 to 14 milliliters etchant is delivered at a rate ofabout 0.25 to 2 milliliters/second (more preferably about 0.3 to 0.5milliliters/second) for a 200 millimeter wafer. The amount delievereddepends on the film thickness to be removed, the concentration ofchemical etchant, rotation rate and etchant temperature.

In a second, preferred, embodiment, the etchant is delivered in stages:a high flow rate stage followed by a lower flow rate stage.Alternatively, all the etchant may be delivered in the lower flow ratestage. In other words, the high flow rate stage is not performed. Thisembodiment is advantageously performed with a nozzle orifice diameter ofabout 0.016 to 0.017 inches.

Assuming that the high flow rate stage is performed, it involvesdelivering etchant to the wafer at a flow rate of about 0.5 to 1.5cc/seconds for a duration of at most about 1 second. This stage helpsbreak up surface tension resistance to wetting, forcing wetting at theedge of the wafer.

In the lower flow rate stage of the second embodiment (which may be theonly stage), the etchant is delivered at a rate of about 0.15 to 0.35cc/second (preferably about 0.3 cc/second) for about 20 to 60 seconds.In a specific embodiment (optimized for 1 micrometer thick films), about8 cc of etchant is delivered over a period of approximately 25 seconds(which may vary depending upon flow rate). In another specificembodiment (optimized for 2 micrometer thick films), about 14 cc ofetchant is applied over a period of about 45 seconds (which may varydepending upon flow rate).

During EBR, some etchant may flow onto the backside of the wafer andetch it. An alternative embodiment for practicing the present inventionis to have the wafer facing upside down, and to apply the etchant to thebackside edge.

After the required amount of liquid etchant has been applied to the edgeof the wafer, deionized water is again applied to the front side of thewafer as a post-EBR rinse 305. This application of deionized water willgenerally continue through the subsequent operations of backside etchingand backside rinsing so as to protect the wafer from any extraneousbackside etchant spray and damage. While the deionized water is applied,the dispense arm moves the etchant nozzle away from the wafer.

At generally about the same time as commencement of step 305, thebackside of the wafer is pre-rinsed 206 with deionized water, which iswet-film stabilized 307 in much the same manner that the front side ofthe wafer was (e.g., the wafer rotation speed is held at about 350 to500 rpm). After the flow of deionized water to the wafer backside ends,a backside etch operation 308 is performed—generally with the sameetchant that was used for the EBR. In a specific embodiment, a thin jet(initially 0.02 to 0.04 inches in diameter) of liquid etchant is aimedat the center of the wafer backside. The etchant is preferably deliveredfrom a tubular nozzle having a diameter of about 0.02 to 0.04 inches anda length of at least about 5 times the diameter. This etchant thendisperses over the entire backside of the wafer. The purpose of the BSEis to remove any residual copper that was formed on the backside of thewafer during formation of the seed layer of PVD copper.The BSE etchantis typically applied using a spray nozzle. Despite gravity, surfacetension generally keeps the etchant in contact with the bottom of thewafer long enough to carry out BSE. Since the chuck arms could interferewith the spraying of etchant on the backside of the wafer, the angle ofthe spray nozzle may be varied during BSE to ensure thorough applicationof the etchant. Because the wafer is generally held up by support pinsthat impinge on the backside of the wafer, the process is generallycarried out at two different speeds to ensure that the etchant flowsadequately over the entire surface. For instance, the wafer may berotated at about 350 rpm during part of the BSE and then rotated at500-700 rpm for the remainder of the BSE. The portions of the backsideblocked by the arms will differ at the two speeds, thus ensuringcomplete coverage. Overall, the BSE process typically takes 1-4 secondsand uses 1 to 5 cubic centimeters of the preferred etchant describedbelow to reduce the concentration of copper on the backside to less than5×10⁻¹⁰ atoms per cm² of substrate.

After BSE, both sides of the wafer (or at least the backside of thewafer) are rinsed with deionized water to rinse any liquid etchant,particles and contaminants remaining from the BSE. See 309. Then theflow of deionized water to the front side ends and about 2 to 4milliliters of a dilute acid, generally less than about 15% by weightacid, is applied to the front side of the wafer to remove residual metaloxide and remove the associated discoloration. See 311. In a specificembodiment, the acid is applied at a rate of about 2 cc/sec. After theacid rinse, deionized water is once again applied to both sides of thewafer, or at least the front side, to rinse the acid from the wafer. Ina specific embodiment, the deionized water is applied for about 15-30seconds at about 300-400 milliliters/min. Finally the wafer can be spunand blow-dried, as desired, on both sides with nitrogen. See 312.Generally, any drying step is carried out at about 750-2000 rpm forabout 10 to 60 seconds, and necessitates a clamping for the wafer onceit reaches about 750 rpm. Many embodiments for the clamping mechanismare possible, and some of these are discussed in more detail below.After this processing in the PEM is completed, a robot arm picks up thewafer and puts it in a cassette.

A second, especially preferred, embodiment of the EBR process is nowdescribed with reference to FIGS. 3A and 3B. This embodiment isparticularly effective at eliminating streaking and narrowing the taperof thex wafer. As with the previously described embodiment, this processcan be carried out by a post-electrofill module, such as module 220 ofFIG. 2A. The entire process is conducted at a rotation speed of betweenabout 150-400 rpm. In one specific embodiment, the rotational speed isabout 225 rpm and in another specific embodiment, the rotational speedis about 350 rpm. So initially, the wafer is accelerated up to aprocessing speed of 225 rpm, or whatever speed is chosen for the processat hand. In the context of FIG. 3A, this may be viewed as part ofoperation 301.

As indicated above, the wafer is pre-rinsed with deionized water appliedto the front of the wafer, while the wafer is spun at about 150-400 rpm.See 302. For this second embodiment, the pre-rinse operation isperformed in multiple sub-operations. FIG. 3B depicts thesesub-operations, which correspond to operations 302 and 303 of FIG. 3A.

The pre-wet conditions are driven by two goals: (1) providing a taperwidth of 400 micrometers or in that neighborhood and (2) performingprewetting rapidly so as to maintain a high throughput. Ideally, thefull prewetting operation will take no longer than about 6 seconds (morepreferably not longer than about 4.5 seconds).

The prewet must be sufficiently long so that the edge bevel etch canproceed rapidly and produce the desired 200-400 micrometer taper width.It is believed (although not proven) that the pre-wet operation helpsremoves residual electroplating additives (e.g. accelerators and/orsuppressors) that protect the copper from rapid attack by the edge beveletchant.

The taper is generically defined as the transition from a region withfull metalization (e.g., the inner or active area of the wafer includingall the dies) to a region without metal (e.g., the edge exclusion area).The taper is typically measured from a region of 95% metal thickness to0% metal thickness. With the EBR method as described, the wafertypically starts with a taper of about 700 micrometers or more.

In a preferred embodiment depicted in FIG. 3B, used with conventionalplating chemistries, the pre-wet operation includes three suboperations.Initially, at 321, the system delivers about 400-1000 ml/minutedeionized water for about 2-4 seconds (preferably about 600 ml/minutefor about 2 seconds). Note that this rate may be doubled or increased byother multiple if two of more deionized water nozzles are employed.Thus, one embodiment of this invention employs two or more nozzles.

Second, at 323, the system delivers about 0.5 to 2 ml (preferably about1 ml) of about 4-6% sulfuric acid for about 0.1 to 2 seconds (at about1-4 ml/second—preferably for about 0.5 seconds) and concurrently deliverdeionized water at about 400-1000 ml/minute for about 0.1 to 2 seconds(preferably about 600 ml/minute for about 0.5 seconds). Thissuboperation may employ two separate nozzles/delivery lines. Forexample, referring to FIG. 2A, deionized water is delivered via line 238and sulfuric acid is concurrently delivered through line 250. Note thatthe flow of deionized water may be continuous between suboperations 321and 323—although this is not necessary.

Third, at 325, the system delivers about 400-1000 ml/minute deionizedwater for about 2-4 seconds (preferably about 600 ml/minute for about 2seconds). Again, the flow of deionized water may be continuous—in thiscase between suboperations 323 and 325—although this is not necessary.

During all pre-wet suboperations, the wafer rotational speed ispreferably maintained at about 150 to 400 rpm. (For 200 mm wafers, 225or 350 rpm may be used.) In all embodiments, the length of the pre-wetoperation depends on the plating chemistry and the rotational speed ofthe wafer. If the plating chemistry includes high concentrations ofaccelerators or suppressors, then relatively longer pre-wet times arerequired. If the rotational speed is relatively fast, then shorterpre-wet times are necessary.

The total pre-wet operation is typically carried out for about 4-24seconds at room temperature, with a decrease of about a factor of 2 forevery 10° C. increase in temperature. The parameters of the pre-wettingensure that the entire front side of the wafer is rinsed, including theareas that were excluded from electrofill by the clamshell lip seal.

After these pre-wet operations, the process is conducted in the mannerdescribed above, beginning at block 304. This second embodiment of theinvention will preferably reduce the taper of the wafer to about 200micrometers. In addition to narrowing the taper, the embodiment can alsobe used to remove electrofilled copper, which commonly reachesthicknesses 1 or even 2 micrometers. The thin taper widths provided bythis invention are particularly well-suited for processes includingsubsequent CMP. Further, the processing of this embodiment isparticularly effective at preventing streaks, which are area oflocalized film oxidation on the wafer. Streaking is caused by thepresence of dilute EBR etching fluid landing on unwanted areas of thewafer (typically after contact with the module wall), and is typicallycharacterized by regions of oxidation of 50 Å or more in thickness. Thestreaks can be determined by visual inspection or defect analysisinstrumentation such as an AIT defect analyzer (made by KLA/Tencor). Theaddition of a rinse deflector attached to the module wall can also beused to prevent deflection of the etchant back onto the wafer surface.

Turning again to FIGS. 2A and 2B, some features of the PEM will bedescribed in further detail. First, note that wafer 224 rides on supportpins 285 (located on wafer chuck arms 281) by static friction.Preferably, the support pins 285 are located from about 5 to 20millimeters, more preferably about 5 to 10 millimeters, in from the edgeof wafer 224. The design of the support pins is determined by the needto supply enough friction to 1) keep the wafer from flying off the chuckif it is aligned slightly off center (i.e. when aligned to the toleranceof the specification of the edge bevel removal process), 2) not slip asthe wafer is accelerated (at typically a rate of 50 to 300 rpm/sec (100rpm/sec in a specific embodiment)) from rest to the EBR rotation rate,and 3) not shed or generate particles. As the wafer's rotational speedincreases, however, it reaches a velocity at which the static frictionfrom resting on the pins can no longer constrain it due to smallmisalignments and the associated centripetal force. To prevent the waferfrom flying off chuck 226 at such velocities, clamping cams 287 may beemployed. The design of suitable cams is described below. For now,simply understand that at defined wafer rotational velocities, theclamping cams rotate into a position that locks wafer 224 in place.

Next note that a dispense-arm 283 functions to hold the dispense nozzle256 and move the nozzle into an accurately controlled location over thewafer 224 during the etching step of the process. The dispense-armdesign is not particularly restrictive. It can move down from above thewafer, in from the side, swing in from the edge, rotate down from above,or any combination of these movements. However, the location of thenozzle is preferably reproducibly accurate to within less than about0.05 mm (more typically less than about 0.02 mm) so that the etchedregion is mechanically under control. Any suitable pneumatic actuator,solenoid, motor controlled gear, or servo controlled motor can activatethe arm. The dispense-arm should move the dispense nozzle accurately tothe edge of the wafer and move the nozzle out of the way to allow thewafer to be transferred into and out of the chuck. The materials ofconstruction should be resistant to the particular chemical etchingsolution used. If the preferred etchant disclosed herein is used,certain stainless steels (e.g. 303, 625, 316L etc.), ceramics (Al₂O₃,zirconia), Tantalum, and plastic coated metals (polypropylene,polyethylene, PTFE, PVDF, PPS; poly-phenylene sulfide) are good choicesbecause they will resist chemical attack, and have sufficient mechanicalstrength .(without creep or flow) to maintain the necessary stringentmechanical tolerances. Similar design considerations hold for the waferchuck.

FIG. 4A schematically shows a nozzle 400 delivering etchant to the frontof the wafer for EBR. Relevant parameters for defining the desiredetchant flow regime include (i) the thickness of the fluid stream (L),which is essentially determined by the diameter of the nozzle, (ii) theradius of curvature of the wafer (R), and (iii) the radial velocity ofthe fluid stream (V), which is determined by the radial component of theetchant's exit velocity from the nozzle and to some degree thecentripetal acceleration from rotation of the wafer. In a planecontaining the normal to the wafer, the nozzle 401 may be angled by Adegrees (generally between about 30 to 70 degrees) with respect to thenormal of the wafer. The component of the etchant's exit velocity fromthe slot nozzle in the plane of the wafer is thus the product of thetotal exit velocity and sin(A). The viscosity (μ) and density (ρ) of theetchant fluid also contribute to the flow regime function. The nozzlealso is angled rotationally at 0 to 90 degrees with respect to the wafertangent in the plane of the wafer.

FIG. 4B schematically shows a nozzle at four different orientations withrespect to a wafer. Each orientation differs from the others in itsangle with respect to the wafer's tangent and/or its angle with respectto the wafer's normal (the “z-direction” out of the plane of the page).Considering first the radial/tangential angle (in the plane of thewafer), if the nozzle is angled purely in the radial direction R (90degrees with respect to the wafer tangent), then it will deliver etchantfluid with a radial angle as shown by nozzle 451. And if the nozzle isangled purely in the tangential direction T (0 degrees with respect tothe wafer tangent), then it will deliver etchant fluid with a tangentialangle as shown by nozzle 452. The nozzle can and often is angledsomewhere between the purely radial and purely tangential directions asillustrated by nozzle 453. As explained, the nozzle orientation can alsovary with respect to the wafer normal. Each of nozzles 451, 452, and 453is angled to the same degree with respect to the wafer normal. Nozzle455, however, is more steeply angled toward the normal.

As mentioned, the three flow regimes of interest include (i) edgebeading, where surface tension forces dominate the behavior of thefluid, (ii) viscous flow, where viscous forces predominate, and (iii)inertial, where inertial forces predominate and the fluid tends tospray. Several experimental observations and calculated trends weremade. Larger nozzles and high flow velocities lead to thicker fluidfilms with more inertia, which tend to fly off the edge of the waferrather than wrap around the side and back. Combinations of low flow, lowrotation, and a wide nozzle result in films that bead at the edge, andsporadically weep from the edge to the back, where they fly off inspurts. A high rotation rate results in very short etchant/surfaceexposure times and the etchant flying off the front surface (not wettingthe sides and back). The experiments and calculations indicate that thethickness of the applied etchant stream should be approximately the samesize or smaller than the radius of curvature of the wafer edge. Thereare a range of flow rates (fluid velocities) and rotation rate that areeffective in producing the required viscous flow conditions. Generally,lower flow rates were effective with higher rotation rates and viceversa.

The three flow regimes can be approximately correlated to values of adimensionless number given by μR/VL²ρ. The parameters of thisdimensionless number were discussed above in conjunction with FIG. 4A.Using this dimensionless number, numbers above about 0.001 correspond tothe edge beading regime, numbers below about 0.0001 correspond to theinertial regime, and numbers in between these correspond to the viscousflow regime.

Edge beading is not desirable for practicing the invention because inthis regime the fluid forms in droplets rather than evenly flowing overthe surface of the wafer. In addition, the movement of such droplets issomewhat unpredictable, and they can flow in from the front edge of thewafer, where the fluid is dispensed from the nozzle, back toward thecenter. The inertial regime is undesirable because in this regime thefluid tends to “fly off” the front edge of the wafer, due to the radialcomponent of the fluid's velocity, rather than flowing over the sideedge. This radial velocity is a result of the exit velocity of the fluidfrom the nozzle and to a smaller degree the centripetal acceleration ofthe rotating wafer. The viscous regime is the regime one wishes tooperate in because in this regime the fluid evenly covers the front edgeof the wafer where the etchant is applied. The viscous fluid also flowsover the side edge and to some degree the back edge of the wafer due tothe radial component of the fluid's velocity.

Using typical etchant solutions, such as the preferred H₂SO₄ and H₂O₂solution described below, it has generally been found that the thicknessof the etchant stream as delivered to the wafer should be about the samesize or slightly smaller than the radius of curvature of the wafer.Using the preferred etchant, and processing a standard wafer of 200millimeters diameter, 0.75 millimeters thickness and 0.15 millimeterradius of curvature, the following parameters were found to have workedwell: a nozzle diameter of between about 0.4 to 0.5 mm, a wafer rotationrate of between about 100 and 500 rpm, an exit velocity of between about40 and 400 cm/sec, a angle for the nozzle of 30 to 70 degrees from thenormal of the wafer, and a rotation angle for the nozzle of about 0 to90 degrees with respect to the direction of rotation (the tangentialdirection, T, of FIG. 4B). It is generally desirable to have the nozzlelocated as close to the wafer as mechanically practical. Further, thelocation of the nozzle with respect to the wafer edge, combined with itslocation above the wafer, determines the etching region which isdependent on the particular application. It has been found that the EBRworks well when the nozzle exit is about 0.3 to 5 millimeters above thesurface of the wafer, and about 0 to 5 millimeters inside its outeredge. The nozzle tube generally should be narrow and long enough toensure that the fluid exits in a stream that stays roughly parallelbefore it hits the wafer.

In particularly preferred embodiment, an angle of about 45 degrees fromthe normal (toward the edge of the wafer along a radial line) is used,and about 0-45 degrees radially in the direction of rotation (along aline parallel to the tangent of the wafer), more preferably 25-35degrees, is used. In this case, the nozzle orifice is offset from theedge of the wafer by about 1.5 to 4.5 millimeters. This embodiment isparticularly effective at eliminating streaking and reducing the taperof the wafer. The angular and translational positions of the nozzle canbe controlled using conventional actuators, such as screw actuators.

The rotation rates specified herein were determined throughexperimentation and calculation, specifically for a 200 millimeter waferpart size and specific viscosity of etchant. However, the invention isnot specific to that part size or etchant viscosity. Similar experimentsand calculations can be performed to optimize the nozzle size,viscosity, flow conditions and rotation rates for other wafer sizes. Theappropriate rotation rate for other size wafers can be estimated bymaintaining the same centrifugal acceleration (v²/r). Since thetangential velocity is v=2πωr/60 (ω is the rotation rate in rpm, r isthe wafer radius in cm, v is the velocity in cm/sec), the centrifugalacceleration is given by a_(c)=(2πω/60)²r. Therefore, neglecting viscousforces and time of flight considerations, the appropriate scaling istherefore r₁/r₂=ω₂ ²/ω₁ ².

The nozzle hole diameter should be optimized along with the flowvelocity and rotation rate to apply a continuous film of fluid onto thewafer. Maintaining the nozzle hole diameter over a fixed potion ofnozzle length is necessary to develop an approximately parallel(non-diverging) exiting fluid flow profile. The fluid nozzle impingementimparts a sufficiently large radial velocity component so that the fluidwill rapidly flow around the wafer edge. Preferably, the nozzle shape istubular. In a specific embodiment, the nozzle is tubular and about 0.5to 1 millimeter in length.

In an alternative embodiment, the nozzle has a slot shape. If a slotnozzle is used, its length should be determined with reference to thewidth of the etched region (ring) that is to be produced. The slot widthshould be small enough so as to minimize chemical usage, splashing ofetchant, and beading of the dispensing volume (avoiding the dispensehaving discrete drops rather than a stream). Typical slot nozzles testedthat were found to be effective were about 2-4 millimeters in length,allowing application of etchant over 1-5 mm of the wafer edge. Usefulslot widths were in the range of about 0.1-0.3 millimeters.

Keep in mind that a large tube (dispensing spray diameters approachingthe dimensions of the edge to be etched) could be used, but would not beas efficient as the approaches described here because of the largeamount of fluid needed. The use of the larger flow volume near therotating chuck cams and arms also increases the propensity for splashingback onto the frontside device areas of the wafer as well. The discloseddesign enables controlled dispensing of the etchant from the top of thewafer, over the side, to the back edge, and even controlled removal ofthe metal from the underside edge of the wafer, without physicallytouching the wafer and thereby contacting the active surface.

Various considerations influence the choice of a liquid etchant. Asmentioned above, the liquid etchant should etch the unwanted metalrapidly at room temperature (e.g., >400 Å/sec). But, it should notaggressively attack the mechanical and electrical components of the etchsystem. Nor should it generate dangerous by-products during the etchingreaction. Preferably, the components of the liquid etchant shouldinclude only those materials readily available in normal integratedcircuit manufacturing facilities. Other beneficial properties of aliquid etchant include a long shelf life (preferably withoutstabilizers), a consistent etching rate over time, low cost, andenvironmental friendliness. In cases where the use of the particularetchant chemical(s) are either expense as a raw material or where thetreatment of waste must be minimized, use of heated etchant can beuseful. For example, etch rates of a 5% H₂O₂ and 5% H₂SO₄ can beincreased approximately 2× for every 10 degrees C. in the range of 20 to50 C. Use of an in line heater after the components have been in-linemixed can enable increased etch rates with relatively low concentrationsof chemical and avoid the normally high rate of etchant breakdown(stability) if it were stored in the mixed form at a elevatedtemperature. In short, one aspect of the invention involves (1) storingthe chemicals at ambient temperature (relatively cold), (2) mixing themcold, (3) heating them after they are mixed just prior to delivery, andthen (4) applying them to the wafer to enable accelerated etch ratecapability. Preferably, the liquid etchant includes an acid andoxidizer. Examples of acids that are useful include sulfuric acid,hydrohalic acids, chromic acid and nitric acid. A preferred etchant forcopper EBR is a solution of H₂SO₄ (sulfuric acid) and H₂O₂ (hydrogenperoxide) in water. A preferred composition of the etchant is 1.4% to10% H₂SO₄ by weight (preferably 2.5% to 7.5%) and 3.5% to 7.5% H₂O₂ byweight (preferably 3.5% to 6.5%).

It has been found that this relatively dilute mixture of hydrogenperoxide and sulfuric acid provides an etchant with an excellent rate ofcopper etch. In storage, the etchant maintains a sufficiently high etchrate for over a month. Alternatively, dilute (about 2-15% by weight)acid and peroxide can be stored in separate containers and mixed in asmall tank for short-term storage prior to use, or mixed on-line justprior to their use. In this case the etch rate is found to not changefor over a year (the normal degradation rate of commercially availablestabalized hydrogen peroxide). There is an exothermic release with themixing of dilute (˜10%/wt) acid and peroxide, but it is small at thesedilute concentrations. Either of these mixing approaches is effectiveand preferred, since sulfuric acid is a stable compound, and lowconcentration hydrogen peroxide (e.g., <10%) can be safely stored forover a year with stabilizers well-known in the art. A system and methodfor in-line mixing and delivery of this preferred etchant is describedin U.S. patent application Ser. No. 09/557,695, previously incorporatedby reference. Processing of the etchant after use is not difficult andis generally compatible with waste-treatment methods that are used toprocess copper electroplating solutions as well.

While sulfuric acid and hydrogen peroxide work well in these capacities,the invention is not so limited. Note that if an oxidant other thanhydrogen peroxide is used, some of the precautions described hereinagainst generating oxygen bubbles can be eliminated. Also, if an acidother than sulfuric acid is used, some of the precautions against theexothermic mixing reaction can be eliminated. Two possible, but lesspreferred, etchant includes S₂O₈ ⁻² (peroxydisulfate) and concentratedHNO₃ (˜30% in water), which is described in U.S. Pat. No. 5,486,234,which is herein incorporated by reference in its entirety.

Generally, the liquid etchant should have physical properties compatiblewith the etching system. The viscosity and density should allow easydelivery onto the semiconductor wafer in a desired flow regime (e.g., aviscous flow regime). It has been found that the fluid properties of themost effective etchants are very similar to those of water (e.g.,surface tension, contact angle, and viscosity). The above-describeddilute sulfuric acid/hydrogen peroxide etchant meets this requirement.

What is claimed is:
 1. A method of removing unwanted metal deposited onan edge bevel area of a semiconductor wafer, the method comprising:rotating the wafer; prerinsing the wafer using deionized water and andacid; and delivering a stream of liquid etchant onto the edge of therotating wafer such that the liquid etchant selectively flows over theedge bevel area while in the viscous flow regime, wherein the liquidetchant substantially removes unwanted metal selectively from the edgebevel area.
 2. The method of claim 1, wherein the stream of liquidetchant is delivered onto the edge of the rotating wafer withoutsubstantially contacting any region of the wafer inside of the edgebevel area.
 3. The method of claim 1, wherein the liquid etchant isdelivered from a nozzle positioned proximate to the edge of the rotatingwafer.
 4. The method of claim 3, wherein the nozzle is pointed so thatdelivery of the liquid etchant has an angular component in the directionof rotation of the wafer edge of about 45 degrees.
 5. The method ofclaim 3, wherein the nozzle is pointed so that delivery of the liquidetchant has a radial component away from the center of the wafer andtoward the edge of the wafer of about 25-35 degrees.
 6. The method ofclaim 1 wherein the wafer is rotated at about 150-400 rpm.
 7. The methodof claim 1 wherein the wafer is rotated at about 225 rpm.
 8. The methodof claim 1 wherein the wafer is rotated at about 350 rpm.
 9. The methodof claim 1 wherein the taper width of the wafer is reduced to less thanabout 300 micrometers.
 10. The method of claim 1 wherein the acid isH₂SO₄.
 11. The method of claim 1 wherein the metal is copper.
 12. Themethod of claim 1 wherein the metal is removed at a rate of at least 400Å per second.
 13. The method of claim 1, wherein the liquid etchant isdelivered from a nozzle having an orifice diameter of about 0.016inches.
 14. The method of claim 1, further comprising: (a) storinghydrogen peroxide and sulfuric acid at a first temperature that isambient or approximately ambient; (b) mixing the hydrogen peroxide andsulfuric acid; (c) heating the mixed hydrogen peroxide and sulfuric acidto produce the liquid etchant, wherein the liquid etchant is deliveredto the edge bevel area at an elevated temperature to thereby enableaccelerated etch rate.
 15. A method of removing unwanted metal depositedon an edge bevel area of a semiconductor wafer, the method comprising:rotating the wafer; prerinsing the wafer using deionized water and acid;and delivering a stream of liquid etchant onto the edge of the rotatingwafer such that the liquid etchant selectively flows over the edge bevelarea to substantially remove unwanted metal selectively from the edgebevel area.
 16. The method of claim 15, wherein the liquid etchant isdelivered from a nozzle having an orifice diameter of about 0.016inches.
 17. The method of claim 15, wherein the prerinsing comprises:(i) delivering only water to the wafer surface; (ii) delivering waterand acid to the wafer surface; and (iii) delivering only water to thewafer surface.
 18. The method of claim 17, wherein the water isdeionized water.
 19. The method of claim 17, wherein (i) comprisesdelivering about 400-1000 ml/minute of water per nozzle for about 2-4seconds.
 20. The method of claim 17, wherein (iii) comprises deliveringabout 400-1200 ml/minute of water for about 2-4 seconds.
 21. The methodof claim 17, wherein (ii) comprises delivering about 0.5 to 2 ml of 4-6%sulfuric acid for about 0.1 to 2 seconds, while concurrently deliveringwater.
 22. The method of claim 15 wherein the wafer is rotated at about150-400 rpm.
 23. The method of claim 15 wherein the wafer is rotated atabout 225 rpm.
 24. The method of claim 15 wherein the wafer is rotatedat about 350 rpm.
 25. The method of claim 15 wherein the acid is H₂SO₄.26. The method of claim 15 wherein the metal is copper.
 27. The methodof claim 15, further comprising: (a) storing hydrogen peroxide andsulfuric acid at a first temperature that is ambient or approximatelyambient; (b) mixing the hydrogen peroxide and sulfuric acid; (c) heatingthe mixed hydrogen peroxide and sulfuric acid to produce the liquidetchant, wherein the liquid etchant is delivered to the edge bevel areaat an elevated temperature to thereby enable accelerated etch rate.